1. Field of the Invention
The present invention relates to a method of manufacturing a disazo pigment for widespread use as a yellow pigment. In particular, the present invention relates to a disazo pigment manufacturing method which can provide a superior coupling reaction rate during the coupling reaction process in the manufacturing process, and to a disazo pigment which can be used for printing inks such as offset ink, gravure ink and the like to impart superior transparency, brightness, color strength and gloss thereto.
2. Description of the Prior Art
Disazo pigments are yellow pigments widely used in paints, printing inks, plastic colorings and the like. These pigments are obtained by carrying out a coupling reaction using a benzidine derivative tetrazo compound (hereinafter called "tetrazo") and acetoacetanilide referred to as the "coupler". This coupling reaction is carried out by adding a tetrazo aqueous solution to a weak acidic aqueous slurry containing coupler particles distributed therein.
Tetrazo is chemically unstable and can easily undergo decomposition and condensation. This is particularly severe when conditions are alkaline, in which case reduction takes place immediately. When conditions are strongly acidic, a high stability is obtained, but under these conditions the coupling reaction hardly proceeds at all. For this reason, coupling is carried out under weak acidic conditions, wherein the added tetrazo immediately reacts with all the coupler.
On the other hand, when the conditions are alkaline the coupler readily dissolves, but when the coupling reaction is carried out under weak acidic conditions, the solubility of the coupler is prety bad, and this results in most of the coupler remaining in the solid phase.
Accordingly, the coupling reaction is carried out as a solid-liquid reaction in which a tetrazo aqueous solution is added to a weak acidic aqueous slurry containing coupler particles distributed therein. As a result, for some reason or other the coupler does not readily dissolve, thus making it impossible to obtain a high reaction rate, as well as creating conditions that will cause unreacted coupler to remain in the pigment. Further, this unreacted coupler that remains in the pigment will decompose into an aromatic amine, thus creating conditions that will lead to greater problems, which include degradation of the pigment characteristics, such as a lowering of the color strength and the like.